x2-(3m+n)x+mn=0
△=(3m+n)^2-4mn=9m^2+2mn+n^2=8m^2+(m+n)^2>0
关于x的方程x2-(3m+n)x+mn=0有两个不相等的实数根,设为：x1,x2,
(x1-n)(x2-n)=x1x2-(x1+x2)n+n^2=mn-(3m+n)n+n^2=-2mn